Pirlindole, 2,3,3a,4,5,6-hexahydro-1H-8-methyl-pyrazine [3,2,1-j,k]carbazole, is a tetracyclic compound of the formula I

Pirlindole is a reversible monoamine oxidase A inhibitor being up to date useful as a medicament in the treatment of depression.
Pirlindole has an asymmetric carbon atom which implies that there are two enantiomers, (S)-pirlindole and (R)-pirlindole.
The state of the art teaches several methods for the enantiomeric separation of pirlindole. For example, The Journal of Pharmaceutical and Biomedical Analysis, 18(1998) 605-614, “Enantiomeric separation of pirlindole by liquid chromatography using different types of chiral stationary phases”, Ceccato et al, discloses the enantiomeric separation of pirlindole by liquid chromatography (LC) using three different chiral stationary phases.
Further, The Journal of Pharmaceutical and Biomedical Analysis 27(2002) 447-455, “Automated determination of pirlindole enantiomers in plasma by on-line coupling of a pre-column packed with restricted access material to a chiral liquid chromatographic column”, Chiap et al., discloses the use of a pre-column packed with restricted access material for sample clean up coupled to a column containing a cellulose based chiral stationary phase for separation and quantitative analysis of the enantiomers.
According to the prior art, Chirality 11:261-266 (1999) all attempts to obtain the enantiomers of pirlindole by selective crystallization with optically active acids failed, and it was only possible to obtain at laboratory scale (few grams) as hydrochloride salt, using derivatization technique in conjunction with preparative chromatography.
The characteristics of the process disclosed in the state of the art limit in a definitive way, its implementation on an industrial or semi-industrial scale due to the necessity to use a separation by chromatography on a large scale which makes the process very costly, difficult to implement and with poor reproducibility.
There is thus a need in the art to find new processes which are viable and of easy industrial application for obtaining enantiomers of pirlindole either in its free base form or as pharmaceutically acceptable salts.